Optoelectronic devices generally include light-emitting devices and photovoltaic devices. These devices generally include an active layer sandwiched between two electrodes, sometimes referred to as the front and back electrodes, at least one of which is typically transparent. The active layer typically includes one or more semiconductor materials. In a light-emitting device, e.g., a light-emitting diode (LED), a voltage applied between the two electrodes causes a current to flow through the active layer. The current causes the active layer to emit light. In a photovoltaic device, e.g., a solar cell, the active layer absorbs energy from light and converts this energy to electrical energy exhibited as a voltage and/or current between the two electrodes. Both types of optoelectronic devices often use a layer of transparent conductive oxide (TCO), such as zinc oxide, in the transparent electrode. A common problem to both types of optoelectronic devices is the relatively high electrical resistivity of the TCO, which leads to resistive losses and consequent inefficiencies in the device.
To overcome this, optoelectronic devices have been developed with electrically isolated conductive contacts that pass through the cell from a transparent “front” electrode through the active layer and the “back” electrode to an electrically isolated electrode located beneath the back electrode. U.S. Pat. No. 3,903,427 describes an example of the use of such contacts in silicon-based solar cells. Although this technique does reduce resistive losses and can improve the overall efficiency of solar cell devices, the costs of silicon-based solar cells remains high due to the vacuum processing techniques used in fabricating the cells as well as the expense of thick, single-crystal silicon wafers. This has led solar cell researchers and manufacturers to develop different types of solar cells that can be fabricated less expensively and on a larger scale than conventional silicon-based solar cells. Examples of such solar cells include cells with active absorber layers comprised of silicon (e.g. for amorphous, micro-crystalline, or polycrystalline silicon cells), organic oligomers or polymers (for organic solar cells), bilayers or interpenetrating layers or inorganic and organic materials (for hybrid organic/inorganic solar cells), dye-sensitized titania nanoparticles in an liquid or gel-based electrolyte (for Graetzel cells), copper-indium-gallium-selenium (for CIG solar cells), cells whose active layer is comprised of CdSe, CdTe, and combinations of the above, where the active materials are present in any of several forms including but not limited to bulk materials, micro-particles, nano-particles, or quantum dots. Many of these types of cells can be fabricated on flexible substrates (e.g., stainless steel foil).
A further problem associated with existing solar fabrication techniques arises from the fact that individual optoelectronic devices produce only a relatively small voltage. Thus, it is often necessary to electrically connect several devices together in series in order to obtain higher voltages in order to take advantage of the efficiencies associated with high voltage, low current operation (e.g. power transmission through a circuit using relatively higher voltage, which reduces resistive losses that would otherwise occur during power transmission through a circuit using relatively higher current).
Several designs have been previously developed to interconnect solar cells into modules. For example, early photovoltaic module manufacturers attempted to use a “shingling” approach to interconnect solar cells, with the bottom of one cell placed on the top edge of the next, similar to the way shingles are laid on a roof. Unfortunately the solder and silicon wafer materials were not compatible. The differing rates of thermal expansion between silicon and solder and the rigidity of the wafers caused premature failure of the solder joints with temperature cycling.
A further problem associated with series interconnection of optoelectronic devices arises from the high electrical resistivity associated with the TCO used in the transparent electrode. The high resistivity restricts the size of the individual cells that are connected in series. Consequently, a large number of small cells must be connected together, which requires a large number of interconnects. Arrays of large numbers of small cells are relatively difficult and expensive to manufacture. Further, with flexible solar modules, shingling is also disadvantageous in that the interconnection of a large number of shingles is relatively complex, time-consuming and labor-intensive, and therefore costly during the module installation process.
Thus, there is a need in the art, for a technique for series connection of optoelectronic devices that overcomes the above disadvantages.